1. Field of the Invention
The present invention relates generally to methods and apparatus for calcining gypsum, and in particular to methods and apparatus for continuously calcining natural gypsum, synthetic gypsum, or combinations of natural and synthetic gypsum.
2. Description of Related Technology
Calcium sulfate (CaSO.sub.4) exists in the following forms, which differ based on the amount of water bonded to the calcium sulfate: anhydrite (soluble or insoluble) (CaSO.sub.4), also referred to as anhydrous calcium sulfate; calcium sulfate hemihydrate, (CaSO.sub.4 .multidot.1/2H.sub.2 O), known as stucco, calcined gypsum, or Plaster of Paris; and calcium sulfate dihydrate (CaSO.sub.4 .multidot.2H.sub.2 O), known as gypsum. Gypsum is found in natural gypsum rock and may also be synthetically produced. The properties of calcium sulfate are described in more detail in 4 Kirk-Othmer Encyclopedia of Chemical Technology 812 (4th ed. 1992), the disclosure of which is hereby incorporated by reference.
Synthetic gypsum, also known as by-product gypsum, is typically produced as a by-product in various chemical processes. For example, flue gas desulfurization (FGD) gypsum is a by-product of stack gas scrubbing processes; titanogypsum is produced by neutralizing waste sulfuric acid from the sulfate process used to manufacture titanium oxide pigment; phosphogypsum is a by-product produced when phosphate ore is acidulated to extract phosphoric acid; fluorogypsum is a by-product of the fluorspar acidulation reaction used to produce hydrofluoric acid; citrogypsum is a by-product of a process for producing citric acid; and borogypsum is a by-product of a process for producing boric acid. Synthetic gypsum typically includes more free water (water not bound to calcium sulfate) than natural gypsum. Natural gypsum typically includes about 1%-3 % free water (unless otherwise indicated, percentages herein indicate percent by weight), compared to synthetic gypsum which is typically mechanically de-watered to a slurry containing about 6%-12% free water. Synthetic gypsum also has a sand-like, granular consistency, compared to natural gypsum which is broken into large rocks during mining. Stucco is produced by drying, grinding, and calcining natural gypsum or synthetic gypsum. Drying refers to removal of free water. Calcination refers to the conversion of calcium sulfate dehydrate to calcium sulfate hemihydrate, according to the following chemical reaction: